The kinetic fibrinolytic assay was adapted for use in performing potency assays on Anistreplase product lots. Both Beecham Pharmaceuticals and the National Institutes of Biological Standards and Controls (NIBSC) utilize a fibrin plate method which is labor intensive and time consuming (overnight assay). Since NIBSC does not plan to establish an International Standard for APSAC (Anistreplase), the company has provided samples of their internal reference standard to both NIBSC and CBER as a surrogate standard for potency determination on lots for release. A study was conducted which involved testing of several lots of Anistreplase against the standard and compared the results obtained by both the fibrin plate assay (Beecham and NIBSC) and the kinetic fibrinolytic (clot lysis) assay in plates (Annual Report, 1986). The results indicated that the latter assay was equivalent to the fibrin plate assay in terms of potency values. In addition a study to establish a U.S. Reference for t-PA was completed. The t-PA preparation was manufactured and vialed by Genetech, Inc. and is identical to Alteplase (recombinant t-PA). Ampoules of the 2nd International Standard (I.S.) for t-PA were provided by NIBSC. Four laboratories performed clot lysis assays comparing the reference material with I.S. The results indicated that the assigned potency of the U.S. Reference for t-PA is 2900 IU/vial.